Decay Asymmetry Research Articles

New Physics Effects on Rare Decays B+u → π+ℓ+ℓ−, ρ+ℓ+ℓ− in a Top Quark Two-Higgs-Doublet Model

Using the form factors from light-cone sum rules, we study the branching ratios and forward-backward asymmetries (FBAs) of the exclusive decays B+u → π+ℓ+ℓ− and B+u → ρ+ℓ+ℓ− (ℓ = e,μ) in the standard model (SM) and the top quark two-Higgs-doublet model (T2HDM). From the numerical results, we find that the new physics contributions cannot provide very large enhancement to the branching ratios and the theoretical predictions are in good agreement with the SM ones. The T2HDM effects on FBAs of these decays are small. Precision measurements of the dilepton invariant mass distributions, especially in the lower dilepton mass region, and the FBAs in the decays B+u → π+ (ρ+)ℓ+ ℓ− will greatly help in discriminating among the SM and the new physics models.

Small amplitude effects inB0→D+D−and related decays

Intrigued by a recent Belle result for a large direct $CP$ asymmetry in ${B}^{0}\ensuremath{\rightarrow}{D}^{+}{D}^{\ensuremath{-}}$, we study the effects of a $\overline{b}\ensuremath{\rightarrow}\overline{u}u\overline{d}$ quark transition by combining the asymmetry information with rates and asymmetries in isospin-related decays. Arguing for a hierarchy among several contributions to these decays, including an exchange amplitude which we estimate, we present tests for factorization of the leading terms, and obtain an upper bound on the ratio of $\overline{b}\ensuremath{\rightarrow}\overline{u}u\overline{d}$ and $\overline{b}\ensuremath{\rightarrow}\overline{c}c\overline{d}$ amplitudes. We prove an approximate $\ensuremath{\Delta}I=1/2$ amplitude relation for $B\ensuremath{\rightarrow}D\overline{D}$, and an approximate equality between $CP$ asymmetries in ${B}^{0}\ensuremath{\rightarrow}{D}^{+}{D}^{\ensuremath{-}}$ and ${B}^{+}\ensuremath{\rightarrow}{D}^{+}{\overline{D}}^{0}$. Violations of these relations by Belle measurements, at $1.8\ensuremath{\sigma}$ and $3.6\ensuremath{\sigma}$ respectively, if confirmed, would indicate a possible new physics contribution in $\overline{b}\ensuremath{\rightarrow}\overline{u}u\overline{d}$. Applying flavor SU(3), we extend this study to a total of ten processes, including $\ensuremath{\Delta}S=0$ decays involving final ${D}_{s}$ and initial ${B}_{s}$ mesons, and $\ensuremath{\Delta}S=1$ decays of $B$ and ${B}_{s}$ mesons into pairs of charmed pseudoscalar mesons. The decays ${B}_{s}\ensuremath{\rightarrow}D\overline{D}$ provide useful information about a small exchange amplitude, responsible for a decay rate difference between ${B}^{+}\ensuremath{\rightarrow}{D}^{+}{\overline{D}}^{0}$ and ${B}^{0}\ensuremath{\rightarrow}{D}^{+}{D}^{\ensuremath{-}}$. A method for determining the weak phase $\ensuremath{\gamma}$, based on $CP$ asymmetries in ${B}^{0}(t)\ensuremath{\rightarrow}{D}^{+}{D}^{\ensuremath{-}}$ and the decay rate for ${B}_{s}\ensuremath{\rightarrow}{D}_{s}^{+}{D}_{s}^{\ensuremath{-}}$ or ${B}^{+(0)}\ensuremath{\rightarrow}{D}_{s}^{+}{\overline{D}}^{0}({D}^{\ensuremath{-}})$, is shown to involve high sensitivity to SU(3) breaking.

Precision measurement of the muon decay parametersρandδ

The TWIST Collaboration has performed new measurements of two of the parameters that describe muon decay: ρ, which governs the shape of the overall momentum spectrum, and δ, which governs the momentum dependence of the parity-violating decay asymmetry. This analysis gives the results ρ=0.750 14±0.000 17(stat)±0.000 44(syst)±0.000 11(η), where the last uncertainty arises from the correlation between ρ and the decay parameter η, and δ=0.750 67±0.000 30(stat)±0.000 67(syst). These are consistent with the value of 3/4 given for both parameters in the standard model of particle physics, and are a factor of two more precise than the measurements previously published by TWIST. A new global analysis of all available muon decay data incorporating these results is presented. Improved lower and upper limits on the decay parameter Pμπξ of 0.995 24<Pμπξ≤ξ<1.000 91 at 90% confidence are determined, where Pμπ is the polarization of the muon when it is created during pion decay, and ξ governs the muon decay asymmetry. These results set new model-independent constraints on the possible weak interactions of right-handed particles. Specific implications for left-right symmetric models are discussed.7 MoreReceived 7 July 2008DOI:https://doi.org/10.1103/PhysRevD.78.032010This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Single top quark production at LHC with anomalous Wtb couplings

We investigate single top production in the presence of anomalous Wtb couplings. We explicitly show that, if these couplings arise from gauge invariant effective operators, the only relevant couplings for single top production and decay are the usual γ μ and σ μ ν q ν terms, where q is the W boson momentum. This happens even in the single top production processes where the Wtb interaction involves off-shell top and/or bottom quarks. With this parameterisation for the Wtb vertex, we obtain expressions for the dependence on anomalous couplings of the single top cross sections, for (i) the t-channel process, performing a matching between tj and t b ¯ j production, where j is a light jet; (ii) s-channel t b ¯ production; (iii) associated t W − production, including the correction from t W − b ¯ . We use these expressions to estimate, with a fast detector simulation, the simultaneous limits which the measurement of single top cross sections at LHC will set on V t b and possible anomalous couplings. Finally, a combination with top decay asymmetries and angular distributions is performed, showing how the limits can be improved when the latter are included in a global fit to Wtb couplings.

Examination of flavor SU(3) in B, [formula omitted] decays

We study a relation between the weak phase γ and the rates and CP asymmetries of several Kπ decays of B+, B0, and Bs, emphasizing the impact of the latter measurements. Current data indicate large SU(3) breaking in the strong phases or failure of factorization (including its application to penguin amplitudes) in Kπ modes of B0 and Bs. SU(3) and factorization only remain approximately valid if the branching ratio for Bs→K−π+ exceeds its current value of (5.27±1.17)×10−6 by at least 42%, or if a parameter ξ describing ratios of form factors and decay constants is shifted from its nominal value by more than twice its estimated error.

Recent developments in radiative B decays

We report on recent theoretical progress in radiative B decays. We focus on a calculation of logarithmically enhanced QED corrections to the branching ratio and forward-backward asymmetry in the inclusive rare decay B̄ → Xsl+l−, and present the results of a detailed phenomenological analysis. We also report on the calculation of NNLO QCD corrections to the inclusive decay B̄ → Xsγ. As far as exclusive modes are concerned we consider transversity amplitudes and the impact of right-handed currents in the exclusive B̄ → K*l+l− decay. Finally, we state results for exclusive B → Vγ decays, notably the time-dependent CP-asymmetry in the exclusive B → K*γ decay and its potential to serve as a so-called ‘null test’ of the Standard Model, and the extraction of CKM and unitarity triangle parameters from B → (ρ,ω)γ and B → K*γ decays.

Measurement of radiative processes at NA48

The first part of this paper will review the new results in the radiative decays of charged kaons available from the NA48/2 experiment at CERN: the first measurement of the DE and INT contribution to the decay K± → π±π0γ in the Tπ* region 0 < Tπ* 80 MeV and the first observation of K± → π±e+e−γ decay. Then in the second part of the paper new results on radiative hyperons decays, concerning the measurement of Ξ0 → Λγ decay asymmetry and the first observation of the Ξ0 → Λe+e− decay from NA48/I, will be reported.

Role of the axial vectora1-meson exchange in hypernuclear nonmesonic weak decays

In the meson-theoretical potential model for the study of the nonmesonic decay rates and asymmetries of hypernuclei, for the first time, the axial-vector ${a}_{1}$ meson (${J}^{\mathrm{PC}}={1}^{++},{m}_{{a}_{1}}=1230\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$) is introduced. The ${a}_{1}$ meson is the chiral partner of the \ensuremath{\rho} meson and has been treated in the meson-pair exchange framework as $\ensuremath{\rho}\ensuremath{\pi}/{a}_{1}$ and $\ensuremath{\sigma}\ensuremath{\pi}/{a}_{1}$. This is analogous to the treatment of \ensuremath{\rho} and \ensuremath{\sigma} exchange in our model. The ${a}_{1}$-meson exchange is found to give remarkable modifications of the parity-conserving decay potentials $({}^{1,3}S\ensuremath{\rightarrow}{}^{1,3}S$ and ${}^{3}{S}_{1}\ensuremath{\rightarrow}{}^{3}{D}_{1})$ at short range $r\ensuremath{\leqslant}1$ fm. As a result, the calculated intrinsic asymmetry parameter ${\ensuremath{\alpha}}_{\ensuremath{\Lambda}}$ for ${}_{\ensuremath{\Lambda}}^{5}\mathrm{He}$ becomes very small and positive in good agreement with the recent high-quality experimental data. The calculated small values of ${\ensuremath{\alpha}}_{\ensuremath{\Lambda}}$ are well compared with the data for ${}_{\ensuremath{\Lambda}}^{11}\mathrm{B}$ and ${}_{\ensuremath{\Lambda}}^{12}\mathrm{C}$ within error bars. The inclusion of the ${a}_{1}$ meson also improves the ${\ensuremath{\Gamma}}_{n}/{\ensuremath{\Gamma}}_{p}$ ratios and leads to a consistent explanation for the existing nonmesonic weak decay data of the light \ensuremath{\Lambda} hypernuclei $(A\ensuremath{\leqslant}12)$. The results calculated in the $\ensuremath{\pi}+2\ensuremath{\pi}/\ensuremath{\rho}+2\ensuremath{\pi}/\ensuremath{\sigma}+\ensuremath{\omega}+K+\ensuremath{\rho}\ensuremath{\pi}/{a}_{1}+\ensuremath{\sigma}\ensuremath{\pi}/{a}_{1}$ exchange interaction model are presented together with the estimates without ${a}_{1}$. Also, the derivation of the expression for the proton asymmetry is described in some detail to elucidate the calculation procedures and phase conventions.

Asymmetry of recoil protons in neutron β decay

A complete analysis of proton recoil asymmetry in neutron decay in the first order of radiative and recoil corrections is presented. The possible contributions from new physics are calculated in terms of low energy coupling constants, and the sensitivity of the measured asymmetry to models beyond the Standard Model are discussed.

Unparticles at heavy flavor scales:CPviolating phenomena

Coupling the scale invariant unparticle sector to flavor physics and assuming that it remains scale invariant I investigate its consequences in heavy flavor physics. A characteristic feature of unparticle physics is a $CP$-even phase leading to novel $CP$ violating phenomena. The phase is large, based on the assumption that the unparticle sector is strongly self-coupled. I consider the $CP$ asymmetry in the leptonic decay ${B}^{+}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}\ensuremath{\nu}$ and the hadronic decay ${B}_{d}\ensuremath{\rightarrow}{D}^{+}{D}^{\ensuremath{-}}$, taking into account constraints of branching ratios and time dependent $CP$ asymmetries. It turns out that the $CP$ asymmetry can be very large even for small couplings because the unparticle interaction term has a lower scaling dimension than the four-Fermi weak interaction term. $CP$ asymmetries in leptonic decays such as ${B}^{+}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}\ensuremath{\nu}$ are neither experimentally searched for nor predicted by any other model. I show that the novel $CP$ violation is consistent with the $CPT$ theorem. I identify the $CP$ compensating mode in the unparticle sector and explicitly demonstrate the exact cancellation as demanded by the $CPT$ theorem.

A theoretical overview of hypernuclear weak decay

The physics of the weak decay of hypernuclei is briefly reviewed from a theoretical point of view. Special regard is devoted to the recent progress concerning the determination of the non-mesonic decay widths and the asymmetry parameters. Convincing evidence has been achieved for a solution of the long-standing puzzle on the ratio Γn/Γp. Very recently, it has been shown that the exchange of a ππ pair in the weak mechanism plays a crucial role in explaining the discrepancies between theory and experiment on the decay asymmetries.

In-medium modification and decay asymmetry of ω mesons in cold nuclear matter

We discuss an asymmetry of the decay {omega}{yields}e{sup +}e{sup -} in nuclear matter with respect to the electron and positron energies. This asymmetry is sensitive to the properties of the {omega} meson self-energy and, in particular, it has a nontrivial dependence on the {omega} energy and momentum. Therefore, this asymmetry may serve as a powerful tool in studying the properties of the {omega} meson in the nuclear medium.

Photoproduction of φ mesons from protons and deuterons with linearly polarized photons at LEPS/SPring-8 experiment

Diffractive photoproduction of φ mesons near threshold involves Pomeron exchange, pseudoscalar exchanges and possible exotic components such as glueball and scalar-meson exchange. We measured φ-meson photoproduction from protons and deuterons at forward angles by using linearly polarized photons with Eγ = 1.5–2.4 GeV. The decay asymmetries show dominance of the natural-parity exchange process. The energy dependence of cross sections will help in determining the behavior of the Pomeron exchange near threshold.

S-wave approximation for asymmetry in nonmesonic decay of finite hypernuclei

We establish the bridge between the commonly used Nabetani-Ogaito-Sato-Kishimoto (NOSK) formula for the asymmetry parameter ${a}_{\ensuremath{\Lambda}}$ in the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\Lambda}}p\ensuremath{\rightarrow}\mathit{np}$ emission of polarized hypernuclei, and the shell-model (SM) formalism for finite hypernuclei. We demonstrate that the $s$-wave approximation leads to a SM formula for ${a}_{\ensuremath{\Lambda}}$ that is as simple as the NOSK one and that reproduces the exact results for ${}_{\ensuremath{\Lambda}}^{5}$He and ${}_{\ensuremath{\Lambda}}^{12}$C better than initially expected. The simplicity achieved here is indeed remarkable. The new formalism makes the theoretical evaluation of ${a}_{\ensuremath{\Lambda}}$ more transparent and explains clearly why the one-meson exchange model is unable to account for the experimental data of ${}_{\ensuremath{\Lambda}}^{5}$He.

Forward coherent ϕ-meson photoproduction from deuterons near threshold

Differential cross sections and decay asymmetries for coherent ϕ-meson photoproduction from deuterons were measured for the first time at forward angles using linearly polarized photons at Eγ=1.5–2.4 GeV. This reaction offers a unique way to directly access natural-parity pomeron dynamics and gluon exchange at low energies. The cross sections at zero degrees increase with increasing photon energy. The decay asymmetries demonstrate a complete dominance of natural-parity exchange processes, showing that isovector unnatural-parity π-meson exchange is small. Nevertheless the deduced cross sections of ϕ-mesons from nucleons contributed by isoscalar t-channel exchange processes are not well described by the conventional pomeron model.

Quintessential kination and leptogenesis

Thermal leptogenesis induced by theCP-violating decay of a right-handed neutrino (RHN) is discussed against the background ofquintessential kination, i.e., in a cosmological model where the energy density of the earlyUniverse is assumed to be dominated by the kinetic term of a quintessence field during someepoch of its evolution. This assumption may lead to very different observational consequencescompared to the case of a standard cosmology where the energy density of the Universe isdominated by radiation. We show that, depending on the choice of the temperatureTr above which kination dominates over radiation, all situations between the strong and thesuperweak wash-out regimes are equally viable for leptogenesis, even with the RHNYukawa coupling fixed to provide the observed atmospheric neutrino mass scale∼0.05 eV. For , i.e., when kination stops to dominate at a time which is not much laterthan when leptogenesis takes place, the efficiency of the process, definedas the ratio between the produced lepton asymmetry and the amount ofCP asymmetry in the RHN decay, can be larger than in the standard scenario of radiationdomination. This possibility is limited to the case when the neutrino mass scale is largerthan about 0.01 eV. The superweak wash-out regime is obtained for , and includes the case when Tr is close to the nucleosynthesis temperature,∼1 MeV. In this latter case the efficiency for leptogenesis is strongly suppressed, but canstill explain the baryon asymmetry observed in the Universe when a resonantCP asymmetry of order 1 is assumed. Irrespective ofTr, we always find a sufficient window above the electroweak temperatureT∼100 GeV for the sphaleron transition to thermalize, so that the lepton asymmetry can alwaysbe converted to the observed baryon asymmetry.

CP violation in scatterings, three body processes and the Boltzmann equations for leptogenesis

We obtain the Boltzmann equations for leptogenesis including decay and scattering processes with two and three body initial or final states. We present an explicit computation of the CP violating scattering asymmetries. We analyze their possible impact in leptogenesis, and we discuss the validity of their approximate expressions in terms of the decay asymmetry. In scenarios in which the initial heavy neutrino density vanishes, the inclusion of CP asymmetries in scatterings can enforce a cancellation between the lepton asymmetry generated at early times and the asymmetry produced at later times. We argue that a sizeable amount of washout is crucial for spoiling this cancellation, and we show that in the regimes in which the washouts are particularly weak, the inclusion of CP violation in scatterings yields a reduction in the final value of the lepton asymmetry. In the strong washout regimes the inclusion of CP violation in scatterings still leads to a significant enhancement of the lepton asymmetry at high temperatures; however, due to the independence from the early conditions that is characteristic of these regimes, the final value of the lepton asymmetry remains approximately unchanged.

Combined D0 measurements constraining theCP-violating phase and width difference in theBs0system

We combine the D0 measurement of the width difference between the light and heavy B-s(0) mass eigenstates and of the CP-violating mixing phase determined from the time-dependent angular distributions in the B-s(0)-> J/psi phi decays along with the charge asymmetry in semileptonic decays also measured with the D0 detector. With the additional constraint from the world average of the flavor-specific B-s(0) lifetime, we obtain Delta Gamma(s)equivalent to(Gamma(L)-Gamma(H))=0.13 +/- 0.09 ps(-1) and vertical bar phi(s)vertical bar=0.70(-0.47)(+0.39) or Delta Gamma(s)=-0.13 +/- 0.09 ps(-1) and vertical bar phi(s)vertical bar=2.44(-0.39)(+0.47). The data sample corresponds to an integrated luminosity of 1.1 fb(-1) accumulated with the D0 detector at the Fermilab Tevatron Collider.

Decay asymmetry in non-mesonic weak decay of light Λ -hypernuclei

The proton decay asymmetry, α p NM , of the polarized Λ-hypernuclei, Λ 5 He, Λ 12 C and Λ 11 B, has been investigated to understand the reaction mechanism of the non-mesonic weak-decay process. These Λ-hypernuclei were produced in the highest statistics ever via the (π +, K +) reaction at 1.05 GeV/c by using the SKS spectrometer at KEK 12 GeV PS. The results show that the α p NM are very small for these s-shell and p-shell hypernuclei.

Unparticle phase effects

Unparticles proposed by Georgi carry $CP$ conserving phases in their propagators. We demonstrate that these peculiar phases have an important impact on $CP$ violation. Without including the strong QCD phases, we study the unparticle phase effects on the direct $CP$ asymmetries in the exclusive decays of ${\overline{B}}_{d}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ and $B\ensuremath{\rightarrow}\ensuremath{\pi}K$, in which the flavor changing neutral currents are forbidden at tree level but induced by one-loop diagrams. Interesting and consistent results comparing to the data are obtained. In addition, we find that unparticles will significantly enhance the differential branching ratio of $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ at the small invariant mass of ${\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$. The forward-backward asymmetries for $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ due to unparticles are also explored.